1. Field of the Invention
The present invention relates to a cartridge adapter for a centrifuge tube.
2. Description of the Prior Art
PCT publication WO 91/06373 discloses a cartridge adapter for supporting a sealed centrifuge tube within a cavity of a centrifuge rotor. The cartridge adapter comprises a pair of matable adapter segments, each of which has an indentation therein. When the segments are joined along mating surfaces the indentation in each segment cooperate to define a recess having a size and shape that closely corresponds (within a certain range of manufacturing tolerances) to the size and shape of some portion or all of the sealed centrifuge tube. When the cartridge adapter is introduced into a rotor cavity one of the two segments, termed the inboard segment A.sub.i, lies radially closer to the axis of rotation VCL. The inboard segment A.sub.i has a keying feature thereon which identifies it as the segment that is introduced into the radially inboard side of the cavity C. The other segment, termed the radially outboard segment A.sub.o, lies radially outwardly of the inboard segment and radially further from the axis of rotation VCL.
FIGS. 1A and 1B are top sectional views, taken in a plane transverse to the longitudinal axis of a prior art sealed plastic centrifuge tube T illustrating the inboard and outboard segments A.sub.i, A.sub.o, respectively, of a cartridge adapter as they support the tube T within a cavity C of a rotor V, illustrated as a vertical angle rotor. At rest, as shown in FIG. 1A, the inside surfaces of the respective inboard and outboard segments A.sub.i and A.sub.o of the adapter lie (within dimensional tolerances) close to or in physical contact with the exterior surface of the sealed tube about its entire circumference. It is noted that the inside surfaces of the adapter segments lie in this disposition with respect to the tube throughout the tube's entire axial length. That is, for each transverse section along the longitudinal axis of the tube, the relationship shown in FIG. 1A is maintained.
With reference to FIG. 1B, as the rotor is spun about the axis of rotation VCL the tube T is exposed both to a radially outwardly directed force F.sub.c and an internal hydrostatic pressure force F.sub.h. The force F.sub.c is caused by the mass of the tube itself under centrifugal load. The force F.sub.h is caused by the mass of the liquid contents of the tube under centrifugal load. In a first portion P.sub.1 of the tube, viz., a portion of the tube T received in the radially inboard segment A.sub.i, the vector sum of the forces F.sub.c and F.sub.h is such that the tube deflects radially outwardly to form a depression indicated by the character D in FIG. 1B. The outer surface of this portion P.sub.1 of the tube is spaced from, or defines a clearance with, the adjacent region R of the inner surface of the inboard segment A.sub.i. This spacing is indicated by the character S in FIG. 1B, while the region R of the inboard segment A.sub.i is indicated by the dot-dash arc. The radial magnitude of the spacing S is dependent upon both the volume, the compressibility of the liquid within the sealed tube, and, in some instances, the fit of the tube T into the adapter.
Throughout the remaining portion of the circumference of the tube T the vector sum of the forces F.sub.c and F.sub.h is such that the exterior of the tube T is forced into intimate contacting relationship with the inner surface of the inboard and outboard segments A.sub.i and A.sub.o of the adapter lying adjacent thereto. The region of intimate contact is indicated by the relatively heavy line of con
As may be appreciated the depression D formed as described has a length dimension measured along the longitudinal axis of the tube. The magnitude of this length dimension is also dependent upon both the volume and the compressibility of the liquid within the tube. If, for example, a tube were completely filled with an incompressible liquid, the force of the liquid would balance the centrifugal force and the depression would likely not form. Since, in practice, the tube is seldom totally filled and liquids are compressible to some extent, a depression D is likely to form along some portion of the length of the radially inward portion of the tube T. The depression may be envisioned as a dimple in the radially inward portion of the tube.
Since the material of the tube in the depression D is spaced from the inner surface of the region R of the inboard segment A.sub.i the hydrostatic force of the liquid within the tube T is not able to provide any support for this region of the inboard segment A.sub.i. The hydrostatic force of the liquid within the tube T does, however, provide some support to that region of the inboard segment A.sub.i into which the tube T has been forced into intimate contacting relationship. As a result, in use, the unsupported region R of the inboard segment of the adapter is deflected radially outwardly.
The magnitude of the deflection is illustrated in FIG. 2 by surface contours of constant deflection or stress. Although FIG. 2 is a vertical perspective taken along a vertical central plane through one half of the inboard segment A.sub.i it should be understood that the surface contours shown in FIG. 2 are symmetric about the vertical central plane. The magnitude of the relative deflections of the region R of the inboard segment A.sub.i of the adapter are indicated by the characters .delta..sub.1 through .delta..sub.4. Since relatively greater deflections produce relatively greater stresses in the adapter segment the surface contours indicating the relative magnitude of the attendant stresses are similar in form to the surface contours of the deflection. The stress contours are thus shown in FIG. 2 and are indicated by the characters .delta..sub.1 through .delta..sub.4, respectively. The character .delta..sub.1 and the character .delta..sub.1 indicate the greatest magnitude of deflection or stress, as the case may be. Those areas of the inboard segment A.sub.i experiencing higher deflections and higher stresses are more likely to fail.
Accordingly, in view of the foregoing it is believed advantageous to provide an adapter in which the areas of relatively higher deflections and higher stresses are eliminated or substantially reduced.
It is common practice in molecular biology applications to utilize a technique termed "equilibrium centrifugation" to isolate various materials in a sample in accordance with their respective density. Typically ethidium bromide, a mutagen, is intercalculated in the material of interest. After centrifugation, with the tube at rest and upright, the materials reorient to form transverse bands at longitudinally spaced locations along the axis of the tube. The ethidium bromide when exposed to ultraviolet light absorbs the ultraviolet light and emits a visible fluorescent light which facilitates location of the band having the material of interest.
One prevalent method to withdraw a band having a material of interest is to hold the tube in a position so that ultraviolet light can shine therethrough to locate the band. The tube may be held by hand or clamped to a stand. A vent needle is inserted into the tube above that band (usually near to the top of the tube). The operator must steady the tube (whether or not it is clamped) while a syringe is then inserted into the tube. The syringe is inserted such that the tip thereof lies immediately beneath the band of material of interest enabling that band to be withdrawn. This process is repeated for each band having a material of interest.
This technique is perceived to exhibit a variety of drawbacks. The operator is exposed to the risk of puncture while handling the tube during the withdrawal process. Further, as should be readily apparent, once the syringe is removed from the tube, liquid from the tube leaks through the punctured opening therein the tube. Typical expedients for preventing such leaks include taping the tube wall or closing the puncture with the clinician's finger until such time as the tube may be deposited into a pool of inactivating liquid. Since ethidium bromide is a known mutagen physical contact with it should be minimized.
Accordingly, in view of the foregoing it is believed advantageous to provide an adapter that supports the tube during centrifugation and also facilitates removal of material without the perceived disadvantages of the prior art.